1. Field of the Invention
The present invention relates to the manufacture of sheets comprising superimposed layers of thermoplastic or other material, by the simultaneous extrusion of various materials and then by the uniting thereof before they are introduced into a die.
2. Description of the Prior Art
In order to unite the materials while avoiding their mixture, they are introduced in coaxial, or at least parallel, flows into the inlet passageway of a die. The use of coaxial cylindrical flows permit the obtainment of a sheet comprising an inner layer and at least one layer enveloping this inner layer. The sheet is always constituted by a sandwich material in which the layers are disposed symmetrically or otherwise relative to the inner layer.
In order to overcome this, it has already been envisaged to dispose in the path of the flows radial partition walls so as to divide the outer flows and permit the injection of different materials on each side of the inner layer. In other cases, parallel conduits separated by partition walls are employed in which the various materials are introduced, these materials coming into contact with one another only at the end of a given parallel path so as to avoid any risk of turbulent flow and mixing.
The document FR-A-No. 2 437 289 of the assignee of the present invention discloses a feed device of a die for manufacturing multi-layer sheets of thermoplastic material which is connected to at least two thermoplastic material extruders and in which a plurality of flows of material are united in a central passageway connected to the inlet of the die. This device comprises an assembly of juxtaposed elements defining the central passageway, an inlet unit disposed at one end of said assembly and in which are formed passageways respectively connected to the extruders, each element comprising at least one radial channel communicating with the central channel and with one of the passages of the inlet unit, optionally through passageways formed in the elements located on the downstream side relative to the flow of the material.
With this arrangement, each element of the assembly of juxtaposed elements is assigned to a layer or well-determined layers of the multi-layer roll of material introduced into the die.
This arrangement implies a successive arrival of each of the flows at this central roll of material which will itself be finally introduced into the die.
The regularity and the homogeneity of the roll delivered to the die depends in particular on those of the extruders, on the relative viscosities of the plastic materials employed and also on the dimensions of the various feed channels and passageways. Further, the contours of the various passageways and channels determine to a large extent the cross-sectional shape of the stream of material delivered to the main roll of material in the central channel.
Consequently, the cross-sectional shape of the various streams of the issuing roll of material must be previously chosen by a suitable dimensioning of the passageways and channels.
Further, in order to adapt a given element to a given material delivered by the extruders, the shape of the radial passageways of the corresponding element must be changed. Consequently, these adapting operations must be carried out before the operation of the plant by dismantling the device.
During operation, the distribution of the material of each layer on the surface of the adjacent material may be corrected by a modification of the relative viscosity of the materials by adjustment of the temperature of the materials introduced.
Now, the temperature-adjusting range is narrow, since, for an excessively low temperature, there is a risk of clogging the radial passageway of the rings, and, for an excessively high temperature, there is a risk of burning the material. Moreover, the reaction time following on each temperature change is relatively long, so that merely using this means of adjustment in operation does not result in a satisfactory precision of the contour of the various layers in a rapid and easy manner.